Phase-change materials, or "PCMs" are compositions that absorb and emit large amounts of heat by changing from one physical state to another, most often reversibly. Typical PCMs change from solid to liquid and back. Paraffinic hydrocarbons are commonly used as PCMs that melt at a desired temperature or over a desired temperature range, as disclosed, for example, in U.S. Pat. No. 5,637,389.
Phase-change materials find use or have been proposed for use in apparel for thermal management, either as discrete layers (see U.S. Pat. No. 5,499,460), as foam additives (see U.S. Pat. No. 5,637,389), as fabric coatings (see U.S. Pat. No. 5,366,801), and as additives to fibers (see U.S. Pat. No. 4,756,958). Where not used as discrete layers, PCMs may be encapsulated to minimize loss of their integrity, dissipation, or evaporation. Encapsulated PCMs are available as microcapsules ("MicroPCMs") having average diameters typically in the range of a few to a few hundred microns. MicroPCMs have been prepared for use as additives to heat exchange fluids, including motor oil (see U.S. Pat. No. 5,141,079).
PCMs which contain paraffinic hydrocarbons are typically flammable. This flammability is often an unacceptable condition. In the past, these difficulties have been overcome by either avoiding problematic physical environments, or by modification of the environments. This approach severely limits the applicability of MicroPCMs in many desirable applications. One approach to this particular problem is found in U.S. Pat. No. 5,435,376, which describes MicroPCMs which include flame resistant halogenated paraffins.
Sol-gel methods are generally covered in Sol-Gel Science--The Physics and Chemistry of Sol-Gel Processing, C. Jeffrey Brinker & George W. Scherer, 1st Ed., Academic Press, (1990), which is hereby incorporated by reference in its entirety.
SUMMARY OF THE INVENTION
The invention provides microencapsulated phase-change materials ("MicroPCMs") that have improved solvent resistance, increased hardness and reduced flammability, and methods for making them. The MicroPCMs of this invention are MicroPCMs coated with a glass-like gel material. They can be produced through sol-gel processing of MicroPCMs. The gel coating provides a degree of physical and flame protection for the microcapsules. The oxidative resistance of such particles is improved significantly, since the coating is believed to minimize oxygen contact with the microcapsules. This can result in reduced incidence of oxidation from atmospheric oxygen for the contents of the microcapsules, e.g., reduced flammability for flammable contents, or reduced chemical oxidation for atmospheric oxidation sensitive contents. The contents contemplated for use in the gel-coated microcapsules include temperature stabilization materials, optical materials or magnetic materials, to name a few examples.
The invention further provides a method for providing a gel coating on MicroPCMs by a sol-gel process. This method involves creating a sol, allowing the gel reaction to take place, mixing MicroPCMs into the sol, and spraying the mixture into, or pouring the mixture onto, a collection area. Curing of the gel takes place in the collection area, and the gel-coated MicroPCMs are collected and ready for use.
The invention further provides a two-component heat transfer slurry, which can be circulated for heating or cooling purposes in a heat transport loop in which heat is transported from a heat source to a heat sink with the use of a pump.
The method of this invention has applicability beyond MicroPCMs. It can be used to coat microcapsules containing optical or magnetic materials, for example. Accordingly, this invention also includes gel-coated microcapsules containing such materials.
As used herein, a "non-released material" is a substance or mixture of substances, which interact with the environment surrounding the microcapsules primarily indirectly. Such materials are not designed to escape the microcapsule under normal operating conditions. Such materials are to be contrasted with others such as flavorings, fragrances, cosmetic products, pharmaceutical or other medical products, which are contained in microcapsules as a means of controlled release of such substances. As used herein, the term "microcapsule" means a unicellular, hollow particle, that is a particle having a peripheral wall or shell enclosing or surrounding a single, hollow cavity, space or void within the interior of the particle which, unless otherwise noted, can be evacuated or filled with a gas, liquid, or solid, such particle being so small as to require means such as an optical microscope for purposes of measuring the dimensions thereof. A microcapsule characterized herein as "spherical" is one which has the shape of a true sphere or spheroid, that is, like a sphere, e.g., oblate or prolate. A microcapsule characterized herein as "porous" is one whose wall has interconnected submicroscopic pores or passages and is permeable to liquid and/or gas, whereas an "impermeable" microcapsule is one whose wall is sealed, non-porous or closed or that has submicroscopic pores or passages so small as to minimize transmission of liquid and/or gas and to maintain the contents within the interior void of the microcapsule. The term "metal" refers to elements which are the central and ligand-bearing atoms in the gel precursors of the invention. As such, it refers not only to those elements commonly referred to as metals, but also the elements such as boron, silicon and germanium which are not traditionally considered metals. The term "potting compounds" refers to semiflexible epoxies with thixotropic and curing agents added, used to affix and cool electronic components. The term "optical material" refers to a material that undergoes physical or chemical changes upon the absorption of photons. The physical changes include the emission of light and/or heat and the reflection of light. Metastable excited states can also be formed. The chemical changes include chemical bond formation and breakage, including various rearrangements, isomerizations and the formation of metastable chemical states.
Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention suitable methods and materials are described below. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specification including definitions, will control. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting.